The present invention relates to an apparatus and method for plasma etching. More particularly, it relates to an apparatus and method for plasma etching wherein a target film is etched by a plasma generated with a high-frequency induction field produced by a spiral coil.
With the increasing miniaturization of a semiconductor integrated circuit element in recent years, exposing light with a shorter wavelength has been used in a lithographic step. At present, a KrF excimer laser (with a wavelength of 248 nm) or an ArF excimer laser (with a wavelength of 193 nm) is used.
As the wavelength of exposing light becomes shorter, the reflectivity of light reflected from a substrate after exposing a resist film becomes higher so that the reflected light from the substrate is more likely to cause variations in the size of a resist pattern.
To prevent the reflected light from being incident on the resist film, there has recently been used a process wherein an organic bottom anti-reflective coating (hereinafter referred to as ARC) is formed under the resist film. The ARC process is a technique used primarily in the process of manufacturing a semiconductor element in a high-performance device with design rules whereby a gate width is 0.25 xcexcm or less.
In the ARC process, it is necessary to etch the ARC after a resist pattern is formed by a conventional lithographic technique. Of a variety of plasma etching apparatus used to etch the ARC, an inductively coupled plasma (ICP) etching apparatus having a spiral coil is used frequently.
As examples of the inductively coupled plasma etching apparatus having a spiral coil, an inductively coupled plasma etching apparatus having a planar coil (see U.S. Pat. No.4,948,458), an inductively coupled plasma etching apparatus having a domed coil (see U.S. Pat. No. 5,614,055), and the like are known.
Referring to FIGS. 5(a) and 5(b), a conventional inductively coupled plasma etching apparatus having a planar single spiral coil will be described.
As shown in FIGS. 5(a) and 5(b), a sample stage 102 as a lower electrode to which high-frequency power is applied is disposed in the lower portion of a grounded chamber 101 having an inner wall covered with an insulator such as ceramic, alumina, or quartz. A semiconductor substrate 103 as a sample to be etched is placed on the sample stage 102. A gas inlet port 108 for introducing etching gas into the chamber 101 via a mass flow controller is provided around the sample stage 102, while a gas outlet port 105 connected to a turbo pump for adjusting pressure in the chamber 1 to the order of 0.1 to 10 Pa is provided in the bottom portion of the chamber 101.
A slide valve 109 having a valve seat and a valve element which rotates relative to the valve seat is provided between the sample stage 102 and the gas outlet port 105 to adjust the amount of gas exhausted from the gas outlet port 105 with the rotation of the valve element. As the slide valve 109, there can be used, e.g., a known slide valve commercially available from VAT Holding AG, Switzerland under the trade name of VAT: Series65 (see Japanese Unexamined Patent Publication Nos. 9-178000 and 9-210222).
An inductively-coupled single spiral coil 104 is provided over a quartz plate 101a as the ceiling of the chamber 101, i.e., over the chamber 101 in opposing relation to the sample stage 102. A coil portion 104a of the single spiral coil 104 has an outermost end A connected to a high-frequency power supply source 106 via a power-supply-side withdrawn portion 104b including a matching circuit (not shown) and an innermost end B connected to a ground source 107 composed of a wall portion of the chamber 101 via a ground-side withdrawn portion 104c. 
When high-frequency power is supplied from the high-frequency power-supply source 106 to the single spiral coil 104, a high-frequency induction field is generated around the single spiral coil 104, which changes the etching gas introduced into the chamber 101 into a plasma. The etching gas that has been changed into the plasma is guided by the high-frequency power applied to the sample stage 102 to a target film on the semiconductor substrate 103 held by the sample stage 102, thereby etching the target film.
By using the conventional inductively-coupled plasma etching apparatus and etching gas composed of a mixture of N2 gas and O2 gas, the present inventors performed an etching process with respect to the ARC as the target film, while holding the pressure in the chamber 101 constant and varying the total flow rate of the gas (the sum of the flow rates of N2 gas and O2 gas, i.e., varying the amount of introduced gas and the opening rate of the slide valve 109. The etching conditions are as shown in Tables 1 and 2. As shown in Tables 1 and 2, etching was performed by varying the amount of introduced gas (N2/O2) and the opening rate of the slide valve.
In Tables 1 and 2, ICP represents high-frequency power applied to the single spiral coil 104 and RF represents high-frequency power applied to the sample stage 102. The target film is the ARC 111 formed on the semiconductor substrate 110, as shown in FIG. 6.
As a result of performing etching under the foregoing conditions, the present inventors have found that the inplane uniformity of the etching rate is degraded if the flow rate of the etching gas is increased. The inplane uniformity of the etching rate is defined as the extent of variation of the etching rate across the surface of the target film and expressed as 3"sgr"/xcexcxc3x97100 (%), where "sgr" is the standard deviation of a data value and xcexc is the mean value of the data value. When variations in data value exhibit a normal distribution, 3"sgr" represents a deviation including 99.74% of the data value, as shown in FIG. 7. The following equation 1 shows 3 "sgr" and xcexc specifically.                               3          ⁢          σ                =                  3          ⁢                                                                      ∑                                      i                    =                    1                                    n                                ⁢                                                      (                                          Xi                      -                      μ                                        )                                    2                                            n                                                          Equation        ⁢                  xe2x80x83                ⁢        1            
where
xcexc: mean value=             ∑              i        =        1            n        ⁢    Xi    n
n: number of samples
Xi: i-th data value.
FIG. 8 is a view showing the relationship between the inplane uniformity of the etching rate and the total flow rate of the gas. In FIG. 8, the horizontal axes represent the total flow rate (sccm) of the etching gas and the opening rate (%) of the slide valve and the vertical axis represents the inplane uniformity (%) of the etching rate.
As will be understood from FIG. 8, the inplane uniformity of the etching rate changes when the total flow rate of the etching gas changes and the opening rate of the slide valve changes. Specifically, the inplane uniformity is improved temporarily (variation of the etching rate is reduced) if the opening rate changes from 10% to 20% but the inplane uniformity is degraded again (variation of the etching rate is increased) if the opening rate exceeds 20%.
The degraded inplane uniformity of the etching rate causes variations in the actual amount of etching across the surface of the target film. If the actual amount of etching varies across the surface of the target film, adverse effects are produced such as variations in the characteristics of a FET in the case of forming the gate electrode of the FET by etching.
In view of the foregoing, it is therefore an object of the present invention to prevent the inplane uniformity of the etching rate from changing even if the total flow rate of the etching gas and the opening rate of the slide valve change.
The present inventors have made a through examination on the cause of a change in the inplane uniformity of the etching rate which occurs when the total flow rate of the etching gas and the opening rate of the slide valve change and achieved the following findings.
Under the condition that the pressure in the chamber is held constant, a change in the total flow rate of the etching gas is caused by a change in the opening rate of the slide valve. Therefore, the present inventors have considered that the inplane uniformity of the etching rate changes in response to the change in the opening rate of the slide valve 109 and examined the relationship between the change in the opening rate of the slide valve 109 and the inplane uniformity of the etching rate.
FIGS. 9(a) to 9(d) show changes in the opening rate of the slide valve 109 composed of a plate-like valve element 109a which rotates relative to a ring-shaped valve seat 109b around a rotation axis 109c and changes in the angle of rotation of the valve element 109a relative to the valve seat 109b. As the opening rate of the slide valve 109 increases, a linearly symmetric line L0 which divides the opening 109d of the slide valve 109 linearly symmetrically is tilted such that the righthand end thereof moves upward, while a first line L1 passing through the center C (center of the opening when the slide valve is fully opened) of the valve seat 109b and perpendicular to the linearly symmetric line L0 is tilted such that the upper end thereof moves leftward. In other words, the angle of tilt of the linearly symmetric line L0 from a first reference line LX extending horizontally in FIG. 9(a) (i.e., the angle of tilt of the first line L1 from a second reference line LY extending vertically) is increased.
FIG. 10 shows the relationship between the opening rate (%) of the slide valve 109 and the angle of tilt of the linearly symmetric line L0 from the first reference line LX (angle of tilt of the first line L1 from the second reference line LY).
Hereinafter, of the two regions of the slide valve 109 separated by the first line L1 in FIGS. 9(a) to 9(d), the region (righthand region) containing the opening 109d will be defined as an exhaust-side region and the region (lefthand region) which does not substantially contain the opening 109d will be defined as a counter-exhaust-side region.
In FIGS. 5(a) and 5(b), it is assumed that the coil portion 104a of the single spiral coil 104 is divided into two regions by a second line L2 connecting the outermost end A of the coil portion 104a (connecting portion between the coil portion 104a and the power-supply-side withdrawn portion 104b) to the innermost end B of the coil portion 104a. Of the two regions of the coil portion 104a separated by the second line L2, the region containing a portion connected directly to the outermost end A is defined as a higher-voltage region (where a high-frequency voltage is relatively high) and the region not containing the portion connected directly to the outermost end A is defined as a lower-voltage region (where the high-frequency voltage is relative low).
From FIG. 11(a), it will be understood that the area of the plasma generation region of the chamber 101 corresponding to the counter-exhaust-side region of the slide valve 109 is larger in the quantity of reactive radicals distributed thereover than in the area corresponding to the exhaust-side region of the slide valve 109. From FIG. 11(b), it will also be understood that the area of the plasma generation region of the chamber 101 corresponding to the higher-voltage region of the single spiral coil 104 is larger in the quantity of reactive radicals distributed thereover than in the area corresponding to the lower-voltage region of the single spiral coil 104 because of a high plasma density.
Therefore, if the exhaust-side region of the slide valve 109 coincides with the higher-voltage region of the single spiral coil 104 and the counter-exhaust-side region of the slide valve 109 coincides with the lower-voltage region of the single spiral coil 104, as shown in FIG. 12(a), the quantity of reactive radicals distributed over the plasma generation region of the chamber 101 becomes uniform. On the other hand, if the exhaust-side region of the slide valve 109 coincides with the lower-voltage region of the single spiral coil 104 and the counter-exhaust-side region of the slide valve 109 coincides with the higher-voltage region of the single spiral coil 104, as shown in FIG. 12(b), the quantity of reactive radicals distributed over the plasma generation region becomes non-uniform, so that the quantity of reactive radicals distributed over the former region is smaller than that distributed over the latter region.
The present invention, which has been achieved based on the foregoing findings, provides a uniform quantity of reactive radicals distributed over the plasma generation region of the chamber by causing the higher-voltage region of the spiral coil to most approximately coincide with the exhaust-side region of the slide valve and causing the lower-voltage region of the spiral coil to most approximately coincide with the counter-exhaust-side region of the slide valve.
Specifically, an apparatus for plasma etching according to the present invention comprises: a chamber; a gas inlet port provided in the chamber to introduce etching gas into the chamber; a sample stage provided within the chamber; a spiral coil disposed externally of the chamber in opposing relation to the sample stage, the spiral coil generating a high-frequency induction field and thereby changing the etching gas into a plasma; a gas outlet port provided in a bottom portion of the chamber to exhaust gas from the chamber; a slide valve having a valve element rotating relative to a valve seat and adjusting an amount of gas exhausted from the gas outlet port with the rotation of the valve element; coil holding means for rotatably holding the spiral coil; and rotative driving means for rotating the spiral coil in response to the rotation of the valve element of the slide valve such that a higher-voltage region of the spiral coil approximately coincides with an exhaust-side region of the slide valve.
In the apparatus for plasma etching of the present invention, when the valve element of the slide valve is rotated to change the opening rate of the slide valve, the exhaust-side region of the slide valve shifts but, if the spiral coil is rotated in response to the rotation of the valve element of the slide valve such that the higher-voltage region of the spiral coil approximately coincides with the exhaust-side region of the slide valve, the lower-voltage region of the single spiral coil coincides with the counter-exhaust-side region of the slide valve, so that the quantity of reactive radicals distributed over the plasma generation region of the chamber becomes uniform and therefore the etching rate over the surface of the target film becomes uniform.
A method for plasma etching according to the present invention comprises: a plasma generating step of changing etching gas introduced into a chamber into a plasma with a high-frequency induction field generated by a spiral coil rotatably disposed in opposing relation to a sample stage within the chamber; an etching step of guiding the etching gas that has been changed into the plasma toward a target film to be etched on a substrate held by the sample stage to etch the target film; and a gas exhaust step of exhausting the gas from the chamber through a bottom portion of the chamber, while adjusting an amount of exhaust by rotating a valve element of a slide valve relative to a valve seat, the etching step including the step of rotating the spiral coil in response to the rotation of the valve element of the slide valve such that a higher-voltage region of the spiral coil approximately coincides with an exhaust-side region of the slide valve.
In accordance with the plasma etching method of the present invention, if the valve element of the slide valve is rotated to change the opening rate of the slide valve, the spiral coil is rotated in response to the rotation of the slide valve such that the higher-voltage region of the spiral coil approximately coincides with the exhaust-side region of the slide valve. Consequently, the higher-voltage region of the spiral coil approximately coincides with the exhaust-side region of the slide valve, while the lower-voltage region of the spiral coil approximately coincides with the counter-exhaust-side region of the slide valve. As a result, the quantity of reactive radicals distributed over the plasma generation region of the chamber becomes uniform and therefore the etching rate over the surface of the target film becomes uniform.
Thus, in accordance with the apparatus and method for plasma etching according to the present invention, the etching rate over the surface of the target film becomes uniform so that adverse effects such as variations in the electric characteristics of a semiconductor element of a FET are prevented.